fix: clear price to null when a comparison finds no merchant offer

Several memory sticks (e.g. Crucial CT8G48C40U5, Patriot Signature Line,
Kingston FURY Beast) kept displaying their pc-builder.io catalog price
("€35.99", "€39.99"...) even after clicking "Compare prices" and getting
"No live price found, using catalog price" - an unconfirmed number was
being shown as if it were still valid, which is misleading given
pc-builder.io's catalog is known to go stale (see the earlier RTX 5070
€128 vs €66.89 case).

New clearPriceIfNoOffers(): when ScrapedPartsController#compare finds
zero offers across all 7 supported merchants, the part's price/priceSource
are reset to null, regardless of whether the price previously came from
the pc-builder.io catalog or an earlier verified merchant match - per
explicit product decision, an empty comparison always wins over keeping a
possibly-stale number around.

- ScrapedPartsController#compare now calls clearPriceIfNoOffers() in the
  empty-offers branch, and returns the resulting price in the JSON
  response so the frontend can mirror it exactly
- NewPartSearch.vue: comparePrices() now sets part.price from the
  response's `price` field (mirroring exactly what the backend just
  persisted) instead of only updating it on a cheaper find; removed the
  now-inaccurate "using catalog price €X" message since that price no
  longer exists once compared with no result
- 3 new tests for clearPriceIfNoOffers: clears an unverified
  pc-builder.io price, also clears a previously-verified merchant price
  when no longer confirmed, and is a no-op when there was no price to
  begin with

Verified end-to-end against the reported Crucial CT8G48C40U5 example:
price went from 35.99 (pc-builder.io) to null after a real comparison
found zero matching offers on any merchant.

app/services/part_scraper_service.ts

@@ -415,12 +415,34 @@ async function updatePriceIfCheaper(
   await part.save()
 }
 
+/**
+ * Clears a part's price when a live comparison found no matching offer on
+ * any supported merchant (see ScrapedPartsController#compare), regardless
+ * of where the price previously came from (pc-builder.io catalog or an
+ * earlier verified merchant match). The pc-builder.io catalog price this
+ * part started with is frequently stale (see shouldKeepExistingPrice's
+ * doc for a confirmed example: pc-builder.io showed €128 for a CPU
+ * actually selling for €66.89) — once a user explicitly asks to verify a
+ * price and no merchant confirms it anymore, keeping the old number
+ * around is misleading, so it's dropped to null instead of silently
+ * falling back to "using catalog price €X".
+ */
+async function clearPriceIfNoOffers(part: ScrapedPart): Promise<void> {
+  if (part.price === null) return
+
+  part.price = null
+  part.priceSource = null
+  part.priceCheckedAt = DateTime.now()
+  await part.save()
+}
+
 export {
   scrapeAllCategories,
   scrapeComponentType,
   ensureFreshCategory,
   isCategoryStale,
   updatePriceIfCheaper,
+  clearPriceIfNoOffers,
   shouldKeepExistingPrice,
   CATEGORY_CONFIG,
   CATEGORY_SLUGS,